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Common methods of treating drinking water to remove/reduce the TDS and other harmful ions like Iron, Arsenic and Fluoride are

• Membrane separation
• Oxidation-flocculation-filtration
• Use absorbents such as activated alumina, activated carbon, chemical resins, etc.

Any of the above techniques are used either exclusively or in combination, depending on the proposed usage and the size of the treatment operation. At a household level, it is better to opt for commercial filters.

Osmosis is a particular diffusion case when water molecules cross through a semi-permeable membrane, leaving behind larger molecules. When a semi-permeable membrane separates water of two different concentrations, fresher water will cross the membrane to dilute the concentrated water in normal Osmosis. In Reverse Osmosis (RO), pressure is exerted on the side of the concentrated water to force the water molecules across the membrane to the fresh waterside.

RO is a standard method to reduce TDS and thus reduce many other harmful ions from drinking water, especially in household-level operations (filter). High TDS reduces the effectiveness of the RO process and makes a system almost ineffective with TDS higher than 2000 ppm. Moreover, since RO treats only a small quantity of the total feed water, the remaining more concentrated water can flow away. In addition to RO, household-level treatment filters may also use additional absorbents like activated charcoal, activated alumina, ion exchange resins, etc., to provide additional treatment to the water. To provide bacteriological protection, some filters may also have a provision for exposing the treated water with ultra violate light.  

Iron occurs in drinking water as Ferrous Oxide (FeO) in soluble form. Ferrous Oxide is converted into Ferric Oxide (Fe2O3) through oxidation. Ferric Oxide is insoluble in water and precipitates at the bottom, separated by filtration. Oxidation is achieved simply by passing air through the water. Manganese di Oxide (MnO2) filter bed is used for better oxidation for large-scale operation. For domestic use, water can be kept overnight in a bucket open to the air and the bottom deposit is cloth filtered in the morning before consumption. Regular use of drinking water with Iron up to 0.3 ppm is safe and is harmful beyond 1.0 ppm. 

Arsenic occurs in water as soluble Arsenite (III) and Arsenate (V). As Arsenate (V) is negatively charged and is easier to remove, sometimes pre-treatment may be necessary to oxidize the Arsenite (III) into Arsenate (V). Oxidation is achieved by adding Potassium per Manganate (KMnO4) and Hydrochloric Acid (HCl). Alum or Ferric Sulphate is then added for coagulation and settlement. The co-precipitation is removed by filtration or using a suitable absorbent such as activated alumina, activated carbon, ion exchange resin etc. Arsenic content in drinking water should be less than 0.05 ppm or 50 parts per billion.  

Fluoride comes in groundwater from Fluorite (CaF2). In a community level Fluoride removal plant (Nalgonda technique), Alum (Hydrated Aluminum) is mixed with water accompanied by vigorous stirring to flocculate Fluoride ions under alkaline conditions, for which some lime is also added. Bed of Activated Alumina, a granular and highly porous material, is used to absorb Fluoride ions. Treated water is removed from the top and may be chlorinated by adding bleaching powder. Alum is later recovered by treating the sludge with acids. In household level operation, fluoride content is reduced by keeping the water overnight in a red clay pot or adding some dried power of Drum Stick seeds. The safe limit of Fluoride content in drinking water is up to 1.0 ppm, the maximum permissible limit being 1.5 ppm. 

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